1. Field of the Invention
The present invention relates to a display device and, in particular, to a color display device having a microresonator (microcavity) structure.
2. Description of the Related Art
In recent years, flat panel displays (FPD) having a thin thickness and which allow reduction in size have attracted much attention. Of various FPDs, liquid crystal display devices are used in various devices. Currently, there is intense research and development into light emitting devices (display devices and light sources) in which a self-emitting electroluminescence (hereinafter referred to simply as “EL”) elements, in particular, organic EL display devices which can emit light at various light emission colors and at a high brightness depending on organic compound materials to be used.
Because an organic EL display device differs from a liquid crystal display device which employs a system in which transmissivity of light from a backlight is controlled by a liquid crystal panel which is placed as a light valve in front of the backlight and the organic EL display device is self-emissive as described above, fundamentally, the usage efficiency of light, that is, the extraction efficiency of light to the outside is high, and consequently, light emission of high brightness can be achieved by the organic EL display device.
However, the light emission brightness of currently proposed organic EL elements is not sufficient. In addition, there is a problem in that, when the supplied current to the organic layer is increased in order to improve the light emission brightness, deterioration of the organic layer is accelerated.
As a method for solving these problems, a method can be considered in which intensity of light at a certain wavelength is intensified in an EL display device by employing a microresonator, as described in Japanese Patent Laid-Open Publication No. Hei 6-275381 and in Takahiro Nakayama and Atsushi Tsunoda, “Elements having Optical Cavity Structure”, Molecular Electronics and Bioelectronics Division of Japan Society of Applied Physics, Third Convention of 1993, p. 135-p. 143.
When a microcavity (microresonator) structure is to be employed in an organic EL element, a metal electrode (for example, cathode) which functions as a reflective mirror is provided as an electrode which is on a rear side of the element, a semi-transmissive mirror is provided on a front surface (on the side of the substrate) of the element, and the optical length L between the semi-transmissive mirror and the metal electrode is designed such that the following equation (1) is satisfied.2nL=(m+½)λ  (1)wherein λ is the light emission wavelength. With this structure, it is possible to selectively intensify light at the wavelength λ and to emit the light to the outside. The variable n in equation (1) represents an index of refraction and the variable m represents an integer (0, 1, 2, 3, . . . ).
This relationship can be easily designed when an organic EL display device having a single wavelength as the emission wavelength, that is, a monochrome organic EL display device, is used, or when the display device is used as a surface light source.
However, when a full-color organic EL display device is to be manufactured, the wavelengths to be intensified within one display panel include, for example, 3 colors of R, G, and B. Therefore, light at different wavelengths must be intensified in different pixels. In order to do so, the optical lengths L between the semi-transmissive mirror and the metal electrode must be changed for each pixel depending on the wavelength of light to be emitted.
On the other hand, unlike a semiconductor device used in an integrated circuit or the like, in a display device, the display itself is viewed by a viewer. Therefore, no structure can be actually employed as a display device unless the structure can stably achieve a high display quality in all pixels.
Because of this, although, for example, theoretically, the cavity (resonator) structure as described above can be realized in a full color display device by setting the optical length in each pixel depending on the light emission wavelength, when the pixels are independently manufactured to achieve different thicknesses, the number of processes in the manufacturing is inevitably increased and the manufacturing processes become more complicated, which results in serious degradation of the quality and variation. In particular, because an organic EL display device currently has a problem with respect to the stability of the display quality, if a resonator structure is simply used, the yield is reduced when the display devices are mass-produced and the manufacturing cost is significantly increased. Therefore, application of the microresonator to an EL display device has been only researched and has not yet been commercialized.